chore: fix nix development environment missing libclang (#718)

## What kind of change does this PR introduce?

Explicitly list `libclang` dependency for the flake setup and configure
the `LIBCLANG_PATH` variable to use that dependency for development
using nix flake environment. I originally fixed it using
`llvmPackages_16.libclang`, but it also works with `pkgs.libclang` so I
am changing to this one in the last pushed commit.

## What is the current behavior?

<img width="1851" height="694" alt="image"
src="https://github.com/user-attachments/assets/d12041dc-b1cb-446a-8b97-361ee03377ab"
/>

Author: nelsonmestevao

flake.nix

@@ -130,6 +130,7 @@
                   pkgs.protobuf
                   pkgs.cargo-outdated
                   pkgs.prom2json
+                  pkgs.libclang
                 ]
                 ++ lib.optionals pkgs.stdenv.isDarwin (with pkgs.darwin.apple_sdk; [
                   frameworks.System
@@ -142,6 +143,8 @@
                   libs.libDER
                 ]);
 
+              env.LIBCLANG_PATH = "${pkgs.libclang.lib}/lib";
+
               # Workaround for https://github.com/rust-lang/cargo/issues/5376
               env.RUSTFLAGS = lib.mkForce (lib.optionals pkgs.stdenv.isDarwin [
                 "-L framework=${config.devenv.profile}/Library/Frameworks"